Re: tc: RTM_GETQDISC causes kernel OOPS

[Eric Dumazet <eric.dumazet@gmail.com>]

Le vendredi 21 mai 2010 à 15:42 -0700, Ben Pfaff a écrit :
> Hi.  While working on some library code for working with qdiscs and
> classes I came upon a kernel OOPS.  Originally I came across it with a
> 2.6.26 kernel, but I can also reproduce it with unmodified v2.6.34 from
> kernel.org.
> 
> At the end of this mail I'm appending both an example of the OOPS and a
> simple test program that reliably reproduces the problem for me when I
> invoke it with "lo" as argument.  The program does not need to be run as
> root.
> 
> After the OOPS, a lot of networking and other system functions stop
> working, so it seems to me a serious issue.
> 
> The null pointer dereference that causes the OOPS is the dereference of
> the return value of qdisc_dev() in tc_fill_qdisc() in
> net/sched/sch_api.c line 1163:
> 
>     1161	tcm->tcm__pad1 = 0;
>     1162	tcm->tcm__pad2 = 0;
>     1163	tcm->tcm_ifindex = qdisc_dev(q)->ifindex;
>     1164	tcm->tcm_parent = clid;
>     1165	tcm->tcm_handle = q->handle;
> 
> I am pretty sure about that, because if I add "WARN_ON(!qdisc_dev(q));"
> just before line 1163 then that warning triggers.
> 
> Thanks,

Indeed, thanks for this very useful report !

We could add a check for TCQ_F_BUILTIN flag, or just make 
qdisc_notify() checks consistent for both old and new qdisc

What other people thinks ?

diff --git a/net/sched/sch_api.c b/net/sched/sch_api.c
index fe35c1f..e454c73 100644
--- a/net/sched/sch_api.c
+++ b/net/sched/sch_api.c
@@ -1210,7 +1210,7 @@ static int qdisc_notify(struct net *net, struct sk_buff *oskb,
 		if (tc_fill_qdisc(skb, old, clid, pid, n->nlmsg_seq, 0, RTM_DELQDISC) < 0)
 			goto err_out;
 	}
-	if (new) {
+	if (new && new->handle) {
 		if (tc_fill_qdisc(skb, new, clid, pid, n->nlmsg_seq, old ? NLM_F_REPLACE : 0, RTM_NEWQDISC) < 0)
 			goto err_out;
 	}